Actuators are used in various mechanical devices to control the features and moving parts of these devices. Specifically, an actuator is a motor that is used to control a system, mechanism, device, structure, or the like. Actuators can be powered by various energy sources and can convert a chosen energy source into motion.
For instance, actuators are used in computer disk drives to control the location of the read/write head by which data is stored on and read from the disk. In addition, actuators are used in robots, i.e., in automated factories to assemble products. Actuators also operate brakes on vehicles, open and close doors, raise and lower railroad gates, and perform numerous other tasks of everyday life. Accordingly, actuators have wide ranging uses.
In the field of aeronautics, actuators are used to control a myriad of control surfaces that allow aircraft to fly. For instance, each of the flaps, spoilers, and ailerons located in each wing, require an actuator. In addition, actuators in the tail control the rudder and elevators of an aircraft. Furthermore, actuators in the fuselage open and close the doors that cover the landing gear bays. Actuators are also used to raise and lower the landing gear of an aircraft. Moreover, actuators on each engine control thrust reversers by which a plane is decelerated.
Commonly used actuators fall into two general categories: hydraulic and electric, with the difference between the two categories being the motive force by which movement or control is accomplished. Hydraulic actuators require a pressurized, incompressible working fluid, usually oil. Electric actuators use an electric motor, the shaft rotation of which is used to generate a linear displacement using some sort of transmission.
Although hydraulic actuators have been widely used in airplanes, a problem with hydraulic actuators is the plumbing required to distribute and control the pressurized working fluid. In an airplane, a pump that generates high-pressure working fluid and the plumbing required to route the working fluid add weight and increase design complexity because the hydraulic lines must be carefully routed. In addition, possible failure modes in hydraulic systems include pressure failures, leaks, and electrical failures to servo valves that are used to position control surfaces. However, one inherent feature of hydraulic systems is that hydraulic flight control systems can use damping forces to maintain stability after a failure has been detected.
Electric actuators overcome many of the disadvantages of hydraulic systems. In particular, electric actuators, which are powered and controlled by electric energy, require only wires to operate and control. However, electric actuators can also fail during airplane operation. For instance, windings of electrical motors are susceptible to damage from heat and water. In addition, bearings on motor shafts wear out. The transmission between the motor and the load, which is inherently more complex than the piston and cylinder used in a hydraulic actuator, is also susceptible to failure. In both electrical and hydraulic systems a mechanical failure of an actuator, e.g. gear or bearing failure, etc., can result in a loss of mechanical function of the actuator. In addition, electrical systems can fail. One type of electrical failure occurs when there is a failure of the command loop that sends communications to an actuator. Another type of electrical failure occurs when a power loop within the actuator fails, such as a high power loop to a motor.
As electronic actuator systems are increasingly used in aircraft designs, new approaches are needed to address possible failure modes of these systems. Fault-tolerance, i.e., the ability to sustain one or more component failures or faults yet keep working, is needed in these systems. Because electric flight control systems do not have hydraulic fluid available for damping, there is a need for alternative fail safe systems that can be used in the event of a failure.